1. Field of the Invention
This invention relates to radiation-hardenable resins useful in paint and varnish applications.
2. Discussion of the Background
Radiation-hardenable resins possess characteristics which make them useful in paint and varnish applications. The current state of development of such resins provides a hardened film having good mechanical properties. And the coating produced possesses good weather and chemical resistance.
The amount of monomers in these resins may be small or large, depending on the viscosity desired and the intended application. The monomers referred to are low molecular weight components--so-called "reactive thinners". Examples are vinyl acetate, vinylpyrrolidone, and alkyl acrylate esters.
These systems however have disadvantages in coating absorptive or porous substrates. These disadvantages include (1) the possibility that the resin or some of its components penetrate into the substrate, and (2) the hardening of the absorptive substrate after hardening of the resin. In the case of highly porous substrates, the portions of liquid which penetrate deeply do not become polymerized in the radiation-hardening step, or they become incompletely polymerized. They remain in liquid form in the substrate.
These low molecular weight "reactive thinners" find use in adjusting the viscosity of the resin and regulating the layer thickness. They are used in amounts of 20-70 wt. % along with higher molecular weight resin components. In general, they present health hazards or have objectionable odors.
Even when all of the components of the resin system are polymerized to a high degree, i.e., when the "reactive thinners" are incorporated into the polymerized system, the hardened film or coating will often retain the characteristic odor of the starting resin, even after hardening. This odor is essentially that of the "reactive thinners" and is perceived as objectionable. Also, it is difficult to employ resins containing "reactive thinners" to produce thin films having a thickness after hardening of 5 or 10 microns unless additional conventional solvents are incorporated. These are evaporated prior to the radiation hardening. Accordingly, the problem presented is to reduce or eliminate these disadvantages.
Reducing the quantity of "reactive thinners" used by adding water to these compositions leads to water-in-oil dispersions which, after hardening, yield serviceable films. A disadvantage with this approach however is that the oil-in-water dispersions formed are not stabile, and that the "reactive thinners" are water-thinnable only to a limited degree. Another disadvantage is that after physical drying the films formed are initially tacky, objectionable odors are produced, and some of the "reactive thinners" are lost by evaporation.
Obtaining direct molten dispersions of acrylic resins in water without using low viscosity monomers is difficult. Such directly dispersed molten resins have high viscosities. And there is a risk of thermaI activation of the acrylic components, i.e. premature polymerization, since such systems are susceptible to thermal activation at processing temperatures above 100.degree. C.
Accordingly, the technological solution of these problems requires the use of classical solvents (e.g., acetone) acting as diluents and suppressing the premature activation problems. After transfer of the system into an aqueous phase, the acetone components are removed by distillation, yielding a radiation-reactive aqueous dispersion which is free of both "reactive thinners" and organic solvents.
In view of the characteristics desired, urethane acrylic monomer systems are candidates for use in these systems. E.g., polyisocyanates can be partially acrylated with the aid of hydroxyalkyl acrylates. The resin character of the urethane acrylate is affected by additional chemical reactions with polyols, to establish the set of characteristics of the hardened film.
By incorporating a certain proportion of acid groups (e.g., --COOH, --SO.sub.3 H, etc.), these resins can be water-dispersed with the aid of alkali hydroxides (e.g., NaOH, KOH). Also, such resins can be water-dispersed with the aid of inorganic or organic acids, by incorporating a certain proportion of tertiary amines (hydroxyamines, etc.). After removal of the acetone by distillation, a solvent-free aqueous solution or dispersion should theoretically remain. But it has turned out, however, that resins produced by this scheme coagulate, either prior to the removal of the acetone or a few hours after the preparation of the dispersion. Many aqueous dispersions form solid, dry, matte films, and the dispersions coagulate after a short time (due to the fact that there are emulsified as well as suspended portions present).
When one starts with aliphatic diisocyanates, such as 1,6-hexanediisocyanate (HDI), methyl-1,6-hexanediisocyanate, or trimethyl-1,6-hexanediisocyanate, stagewise acrylation, urethane-formation, and emulsification do not yield a stabile emulsification of the resulting resin. If triols are used as co-components, the resin gelatinizes, and the triols are expelled to form suspended elements of the emulsion, with subsequent paste-formation. When the film dries it forms a cracked, matte layer which is unusable.
There is thus a strongly felt need for a storage-stable, radiation-hardenable, NCO-free aqueous emulsion not suffering the above disadvantages.